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Application of plasma cleaning equipment in printed circuit board


Printed circuit boards (PCBs) are the providers of electrical connections for electronic components and are the core components of electronic devices that have been widely used. The use of printed circuit boards can significantly reduce wiring and assembly errors and improve automation and production efficiency.

In recent years, with the telecommunications industry, especially communication products, computers and components, semiconductors, liquid crystal and optoelectronics industries such as ultra-precision industrial cleaning equipment and high value-added equipment accounted for the proportion of increasing, plasma cleaning equipment has become the basic equipment for many electronic information industry.

About plasma cleaning


Plasma is a state of matter, also known as the fourth state of matter, in the same way as a solid, a liquid or a gas. The plasma state is when enough energy is applied to a gas to dissociate it. The “active” components of the plasma include: ions, electrons, activated groups, excited nuclei (sub-stable), photons, etc.. Plasma cleaning equipment is the use of these active components of the micron on the surface of the sample treatment, to clean, modify, photoresist and other effects.

However, PCB has the disadvantages of dirty surface, poor wettability and insufficient adhesive properties, which limits its application in bonding, printing, etc. Therefore, it is necessary to surface treatment of PCB, the main methods are chemical wet treatment, plasma treatment, etc.

Chemical and physical reaction in plasma process

There are two main types of reactions involved in the process: a chemical reaction, mainly using H2, O2, N2, CO2, CF4, air, etc. as the medium, which generates highly reactive free radical particles after high pressure ionization, and a redox reaction with the material on the material surface to generate new substances and be discharged from the material surface.

The other is a physical reaction, using inert gases such as Ar and He as the medium. The positive ions, electrons and other high-energy particles generated by these gases after ionization can continuously impact the material surface until the contaminants are removed from the material surface.

Advantages of plasma cleaning equipment

1. Environmental protection technology: plasma method for gas-solid phase dry reaction, no water consumption, no need to add chemical reagents, no pollution to the environment, can replace the chemical treatment process of sodium salt solution.

2. Low temperature: close to room temperature, especially suitable for polymer materials, involving only a shallow layer (10-1000A) on the surface of polymer materials, less destructive compared to chemical methods.

3. Low price: simple equipment, easy operation and maintenance, continuous operation, often with a small bottle ‘i body instead of a few kilograms of cleaning solution, no treatment of waste liquid costs, so the treatment costs will be significantly lower than the wet chemical treatment.

A characteristic of plasma cleaning is that often after plasma treatment of the object surface, many new active genes are often generated to activate the object surface and change its properties, which can greatly improve the wettability and adhesion of the object surface, which is crucial for many materials.

Comparison of wettability after PCB surface treatment


As can be seen from the figure, the water contact angle of the untreated PCB surface is about 70°; after chemical wet treatment, the water contact angle is reduced to about 40° and the surface wettability is improved; after low-temperature plasma treatment, the surface water contact angle of the PCB is only about 20° and the wettability is best.

This is because the high voltage ionization of air will produce highly reactive free particles that are electrically neutral, and these particles will generate hydrophilic groups after the redox reaction with the material on the PCB surface, which will enhance the wettability of the PCB surface; in addition, the etching effect incidental to the plasma treatment process will also increase the roughness of the PCB surface, thus increasing the contact area between solid and liquid.

Comparison of roughness after PCB surface treatment


The step height of the PCB surface after chemical wet treatment is about 1,093 nm, and the step height after plasma treatment is around 1,886 nm, indicating that the surface roughness of the PCB is significantly higher in plasma treatment than in chemical wet treatment, which is one of the main reasons for the better wettability of the PCB surface due to plasma treatment.

Effect of process parameters on the effectiveness of plasma treatment

Effect of power

The plasma treatment of PCBs was carried out under different power conditions at 0.2 MPa for 3 s. As seen in Fig. 1, the water contact angle on the PCB surface decreases and then increases slightly with increasing power. After plasma treatment at 500, 600 and 700 W, the water contact angle of the PCB is smaller, i.e., the wettability is better.

It can be seen that the plasma discharge power has a significant effect on the modification of PCBs, and an appropriate increase in power can enhance the modification effect and improve the wettability of PCBs. However, when the power reaches a certain value, the effect of increasing the power on the improvement of PCB wettability does not change much, and may even become worse.

This phenomenon is determined by the mode of action of low-temperature plasma surface treatment: the plasma treatment process may introduce specific functional groups on the surface of the material to play a role in etching, the formation of cross-linked structural layer or generate surface radicals, these effects are generally not a single presence, often dominated by some role, several roles coexist. When the power is too high, the high-energy particles within the plasma increases, the etching effect is enhanced, and part of the grafted functional groups are re-etched away, so that the water contact angle rises slightly.

Effect of air pressure

According to the basic principle of physical-chemical reaction, the reaction is generally proportional to the pressure, with the increase of pressure, the free radicals involved in the reaction become more and the reaction is accelerated. And the lower the gas pressure, the longer the average free range of molecules, the higher the probability of collision between molecules, the more energy accumulated, and the stronger the attack of particles.

Under the condition of certain power, the water contact angle of PCB decreases and then increases as the air pressure increases from 0.1 MPa to 0.6 MPa, and the lowest water contact angle of PCB is about 14.8° with the best wettability when the power is 500 W and the air pressure is 0.4 MPa.

This is because the increase in air pressure means that the plasma density increases. For plasma surface treatment with air as the medium for chemical reaction-led plasma, the increase in plasma density can significantly improve the cleaning rate and effect of plasma. However, a continuous increase in air pressure leads to a decrease in particle energy, which in turn affects the etching effect produced by the particle bombardment to the surface.

The role of plasma cleaning PCB board


1. Hole wall cavitation / removal of hole wall resin drilling dirt.

2. Removal of carbide.

3. Removing the slag of PCB boards in mechanical drilling and laser drilling due to the high temperature that causes the polymer material to melt on the metal surface of the hole wall, especially suitable for laser drilling small holes that are chemically difficult to access.

4. Treating PTFE sheets to improve the bonding strength of the plating.

5. On soft or hard circuit boards, cleaning the surface before laminating and spraying tin to improve the bonding performance.

6. Cleaning gold joints to improve the bonding strength of wires.

7. Electronic parts activation before package sealing or package to diphenyl cladding.

The principle of plasma cleaning PCB board

In the vacuum chamber inside the gas molecules to apply energy (such as electricity), by accelerating the collision of electrons, so that molecules, atoms, the outermost electron is excited, and generate ions, or highly reactive free radicals.The-principle-of-plasma-cleaning-PCB-board
The ions and radicals thus generated are accelerated by the continuous impact and the force of the electric field, so that they collide with the surface of the material and destroy the molecular bonds within a few microns, inducing a certain thickness reduction and generating a concave and convex surface, while forming the functional groups of the gas components and other physical and chemical changes on the surface, improving the copper plating adhesion, decontamination, etc. The gases commonly used for the above plasma treatment are oxygen, nitrogen and carbon tetrafluoride gas.


Plasma treatment is currently mainly used for high frequency boards, HDI, soft and hard combination boards, and is particularly suitable for polytetrafluoroethylene (PTFE) material boards. Productivity is not high, the cost of large is also its disadvantage, but plasma treatment advantages are also obvious, compared to other surface treatment methods, it in the treatment of Teflon activation, to improve its hydrophilic, to ensure that the hole metallization, laser hole processing, removal of precision lines between the residual dry film, roughening, reinforcement pre-treatment, solder resistance and screen printing character pre-treatment, etc. Its advantages are irreplaceable, and also has is clean, environmentally friendly Features.



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